This invention relates to pressure sensitive adhesive compositions which include a copolymer of isoprene and a sulfonate monomer prepared by emulsion polymerization process and about 25 to 300 parts by weight of a hydrocarbon resin of a petroleum or coal tar distillate, aliphatic dienes and mono-and diolefins, cyclic olefins of 5 or 6 carbon atoms and hydrogenated polycyclics per 100 parts by weight of the sulfonated copolymer and about,.5 to about 50 parts by weight of a preferential polar plasticizer such as zinc stearate per 100 parts by weight of the sulfonated copolymer.
Conventional high strength adhesives commonly used in the pressure sensitive adhesive industry are prepared by solvent based techniques. For example, block copolymers based on polystyrene and isoprene, can be dissolved in a suitable solvent containing a hydrocarbon resin tackifier, which upon casting on a variety of substrates yield materials having outstanding adhesive properties. These materials can exhibit tack properties of 2-5 lbs. per inch (180.degree. peel test) and yet retain their tack under load over long periods of time. These properties are normally attributed to systems that display a combination of appropriate polymer modulus coupled with extremely high molecular weight or a suitable network that is induced in the resulting polymer system. These materials have achieved great prominence in the adhesive industry.
Despite the success of solvent based adhesives there is a current need by the adhesive industry to employ either hot melt of water based adhesive candidates. The reasons for the trend to water based PSA's is that the environmental and toxicity restrictions imposed by federal, state, and local governments markedly restrict the use of effective solvents. In addition, the low polymer content in such solvent based adhesives can present a financial liability, when employed on a large scale. As a result of these factors, it is highly desirable that PSA's having similar properties to those achievable from solvent based systems be obtained from water based systems. Unfortunately, there has not been a significant amount of success in achieving the PSA properties from a water based polymer system that can equal the properties that one needs from a solvent based PSA. The reasons for that deficiency of the properties of water based PSA's are several: A water based PSA based on a conventional hydrocarbon monomer provides a hydrophobic polymer system contained in an aqueous matrix. The fusion or drying of a water based PSA system with all its incumbent surfactants and other additives to stabilize the emulsions adversely affect the adhesive performance in such systems. Consequently, the commonly emulsified water based PSA's are deficient in their adhesive character and their long-term creep resistance (cohesive strength). An urgent need exists for materials which will fill this void.
The use of hot melt adhesives has also been growing very rapidly in the adhesive industry, particularly for packaging application. The hot melt adhesives are generally applied (as the name implies) by conventional extrusion or coating techniques in the temperature range of 250 to 450.degree. F. on one of the surfaces to be bonded. The other surface is brought in contact with the hot surface for a sufficient period of time for the melt to cool, whereupon solidification a strong and durable bond is formed.
The key requirements of resins suitable for hot melt adhesive applications are that they should have good physical properties, e.g., tensile strength, etc., at ambient conditions, and they can flow easily at fabrication temperatures and they must have sufficiently good compatibility that they do not phase separate on storage or during subsequent use. The ethylene vinyl acetate (EVA) copolymers and styrene block copolymers, such as Kraton, have been extensively used as hot melt adhesives; however, their use has been limited largely to pressure sensitive adhesives.
There is a significant demand in the marketplace today for polymer systems which exhibit good green strength or tensile properties at ambient temperatures which, when heated to a predetermined temperature, will give good flow such that they may be applied to a coating or substrate by melt extrusion or other suitable techniques. Alternatively, the system could be waterborne. That is, the entire adhesive package be in the form of an emulsion which after appropriate coating on a substrate provide good green strength. Such a system should also be easy in its flow characteristics during coating and fusion steps. It is desired from such adhesives that they be easier to handle, if needed to be transferred from one substrate to the other for transfer tape applications. In the past it has been common to employ organic solutions as a way of applying a coating of a polymer system which is designed to have good adhesive properties. As mentioned earlier, this practice is now under considerable pressure, due to the fact that the organic solvents must be evaporated from the coating giving rise to pollution